1. Field of the Invention
The present invention relates to a projection exposure apparatus and method for use to form a pattern of a semiconductor integrated circuit, or a liquid crystal device, or the like.
2. Related Background Art
When a circuit pattern of a semiconductor device or the like is formed, so-called photolithography technology is required. In this process, a method, in which a reticle (a mask) pattern is formed on a substrate such as semiconductor wafer, is usually employed. The surface of the substrate is applied with photosensitive photoresist so that a circuit pattern is transferred to the photoresist in accordance with an image irradiated with light, that is, in accordance with the shape of the pattern corresponding to a transparent portion of the reticle pattern. In a projection exposure apparatus (for example, a stepper), the image of a circuit pattern drawn on the reticle so as to be transferred is projected on the surface of the substrate (wafer) via a projection optical system so as to be imaged.
In an irradiation optical system for irradiating the reticle with light, an optical integrator such as a fly-eye type optical integrator (a fly-eye lens) and a fiber is used so as to uniform the distribution of the intensities of irradiation light with which the surface of the reticle is irradiated. In order to make the aforesaid intensity distribution uniform optimally, a structure which employs the fly-eye lens is arranged In such a manner that the reticle-side focal surface (the emission side) and the surface of the reticle (the surface on which the pattern is formed) hold a substantially Fourier transformed relationship. Also the focal surface adjacent to the reticle and the focal surface adjacent to the light source (the incidental side) hold the Fourier transformed relationship. Therefore, the surface of the reticle, on which the pattern is formed, and the focal surface of the fly-eye lens adjacent to the light source (correctly, the focal surface of each lens of the fly-eye lens adjacent to the light source) hold an image formative relationship (conjugated relationship). As a result of this, irradiation light beams from respective optical elements (a secondary light source image) of the fly-eye lens are added (superposed) because they pass through a condenser lens or the like so that they are averaged on the reticle. Hence, the illuminance uniformity on the reticle can be improved. Incidentally, there has been disclosed an arrangement capable of improving the illuminance uniformity in U.S. Pat. No. 4,497,015 in which two pairs of optical integrators are disposed in series.
In a conventional projection exposure apparatus, the light quantity distribution of irradiation beams to be incident on the optical integrator, such as the aforesaid fly-eye lens, has been made to be substantially uniform in a substantially circle area (or in a rectangular area), the center of which is the optical system of the irradiation optical system.
FIG. 14 illustrates a schematic structure of a conventional projection exposure apparatus (stepper) of the above described type. Referring to FIG. 14, irradiation beams L140 pass through a fly-eye lens 41c, a spatial filter (an aperture diaphragm) 5a and a condenser lens 8 so that a pattern 10 of a reticle 9 is irradiated with the irradiation beams L140. The spatial filter 5a is disposed on, or adjacent to a Fourier transformed surface 17 (hereinafter abbreviated to a "pupil surface or plane") and also referred to as a Foruier transform plane with respect to the reticle side focal surface 414c of the fly-eye lens 41c, that is, with respect to the reticle pattern 10. Furthermore, the spatial filter 5a has a substantially circular opening centered at a point on optical axis AX of a projection optical system 11 so as to limit a secondary light source (plane light source) image to a circular shape. The irradiation light beams, which have passed through the pattern 10 of the reticle 9, are imaged on a resist layer of a wafer 13 via the projection optical system 11. In the aforesaid structure, the number of apertures of the irradiation optical system (41c, 5a and 8) and the number of reticle-side apertures formed in the projection optical system 11, that is .sigma. value is determined by the aperture diaphragm (for example, by the diameter of an aperture formed in the spatial filter 5a), the value being 0.3 to 0.6 in general.
The irradiation light beams L140 are diffracted by the pattern 10 patterned by the reticle 9 so that 0-order diffracted light beam Do, +1-order diffracted light beam Dp and -1-order diffracted light beam Dm are generated from the pattern 10. The diffracted light beams Do, Dp and Dm, thus generated, are condensed by the projection optical system 11 so that interference fringes are generated. The interference fringes, thus generated, correspond to the image of the pattern 10. At this time, angle .theta. (reticle side) made by the 0-order diffracted light beam Do and .+-.1-order diffracted light beams Dp and Dm is determined by an equation expressed by sin .theta.=.lambda./P (.lambda.: exposure wavelength and P: pattern pitch).
It should be noted that sin .theta. is enlarged in inverse proportion to the length of the pattern pitch, and therefore if sin .theta. has become larger than the number of apertures (NA.sub.R) formed in the projection optical system 11 adjacent to the reticle 9, the .+-.1-order diffracted light beams Dp and Dm are limited by the effective diameter of a pupil (a Fourier transformed surface) 12 in the projection optical system 11. As a result, the .+-.1-order diffracted light beams Dp and Dm cannot pass through the projection optical system 11. At this time, only the 0-order diffracted light beam Do reaches the surface of the wafer 13 and therefore no interference fringe is generated. That is, the image of the pattern 10 cannot be obtained in a case where sin .theta.&gt;NA.sub.R. Hence, the pattern 10 cannot be transferred to the surface of the wafer 13.
It leads to a fact that pitch P, which holds the relationship sin .theta.=.lambda./P.congruent.NA.sub.R, has been given by the following equation. EQU P.congruent..lambda./NA.sub.R (1)
Therefore, the minimum pattern size becomes about 0.5.multidot..lambda./NA.sub.R because the minimum pattern size is the half of the pitch P. However, in the actual photolithography process, some considerable amount of focal depth is required due to an influence of warp of the wafer, an influence of stepped portions of the wafer generated during the process and the thickness of the photoresist. Hence, a practical minimum resolution pattern size is expressed by k.multidot..lambda./NA.sub.R, where k is a process factor which is about 0.6 to 0.8. Since the ratio of the reticle side number of articles NA.sub.W and the wafer side number of articles NA.sub.R is the same as the imaging magnification of the projection optical system, the minimum resolution size on the reticle is k.multidot..lambda./NA.sub.R and the minimum pattern size on the wafer is k.multidot..lambda./NA.sub.W =k.multidot..lambda./B.multidot.NA.sub.R (where B is an imaging magnification (contraction ratio)).
Therefore, a selection must be made whether an exposure light source having a shorter wavelength is used or a projection optical system having a larger number of apertures is used in order to transfer a more precise pattern. It might, of course, be considered feasible to study to optimize both the exposure wavelength and the number of apertures.
However, it is so far difficult for the projection exposure apparatus of the above described type to shorten the wavelength of the irradiation light source (for example, 200 nm or shorter) because a proper optical material to make a transmissive optical member is not present and so forth. Furthermore, the number of apertures formed in the projection optical system has approached its theoretical limit at present and therefore it is difficult to further enlarge the apertures. Even if the aperture can be further enlarged, the focal depth expressed by .+-..lambda./2NA.sup.2 rapidly decreases with an increase in the number of apertures, causing a critical problem to take place in that the focal depth required in a practical use further decreases.
In Japanese Patent Publication No. 62-50811 for example, there has been disclosed a so-called phase shift reticle arranged in such a manner that the phase of each of transmissive light beams traveled from specific points in the transmissive portions of the circuit pattern of the reticle is shifted by .pi. from the phase of transmissive light beams traveled from the other transmissive portions. By using a phase shift reticle of the type described above, a further precise pattern can be transferred.
However, the phase shift reticle has a multiplicity of unsolved problems because of a fact that the cost cannot be reduced due to its complicated manufacturing process and inspection and modification methods have not been established even now.
Hence, an attempt has been made as projection exposure technology which does not use the phase shift reticle and with which the transference resolving power can be improved by modifying the method of irradiating the reticle with light beams. One irradiation method of the aforesaid type is a so-called annular zone irradiation method, for example; arranged in such a manner that the irradiation light beams which reach the reticle 9 are given a predetermined inclination by making the spatial filter 5a shown in FIG. 14 an annular opening so that the irradiation light beams distributed around the optical axis of the irradiation optical system are cut on the Fourier transformed surface 17.
In order to establish projection exposure having a further improved resolving power and a larger focal depth, an inclination irradiation method or a deformed light source method has been previously disclosed in PCT/JP91/01103 (filed on Aug. 19, 1991). The aforesaid irradiation method is arranged in such a manner that a diaphragm (a spatial filter) having a plurality (two or four) openings, which are made to be eccentric with respect to the optical axis of the irradiation optical system by a quantity corresponding to the precision (the pitch or the like) of the reticle pattern, is disposed adjacent to the emission side focal surface of the fly-eye lens so that the reticle pattern is irradiated with the irradiation light beams from a specific direction while inclining the light beams by a predetermined angle.
However, the above mentioned inclination irradiation method and the deformed light source method have a problem in that it is difficult to realize a uniform illuminance distribution over the entire surface of the reticle because the number of effective lens elements (that is, the number of secondary light sources capable of passing through the spatial filter) decreases and therefore an effect of making the illuminance uniform on the reticle deteriorates. What is worse, the light quantity loss is excessive large in the system which has a member, such as the spatial filter, for partially cutting the irradiation light beams. Therefore, the illumination intensity (the illuminance) on the reticle or the wafer can, of course, deteriorate excessively, causing a problem to take place in that the time taken to complete the exposure process becomes long with the deterioration in the irradiation efficiency. Furthermore, a fact that light beams emitted from the light source concentrically pass through the Fourier transformed plane in the irradiation optical system will cause the temperature of a light shielding member, such as the spatial filter, to rise excessively due to its light absorption and a measure (air cooling or the like) must be taken to prevent the performance deterioration due to change in the irradiation optical system caused from heat.
In a case where a diaphragm of the aforesaid type is disposed adjacent to the emission side focal surface of the fly-eye lens, some of the secondary light source images formed by a plurality of the lens elements are able to superpose on the boundary portion between the light transmissive portion of the diaphragm and the light shielding portion of the same. This means a fact that the secondary light source image adjacent to the aforesaid boundary portion is shielded by the diaphragm or the same passes through the boundary portion on the contrary. That is, an unstable factor, such as the irradiation light quantity, is generated and another problem arises in that the light quantities of the light beams emitted from the aforesaid diaphragm and that are incident on the reticle become different from one another. Furthermore, in the inclination irradiation method, the positions of the four openings (in other words, the light quantity distribution in the Fourier transformed surface) must be changed in accordance with the degree of precision of the reticle pattern (the line width, or the pitch or the like). Therefore, a plurality of diaphragms must be made to be exchangeable in the irradiation optical system, causing a problem to arise in that the size of the apparatus is enlarged.
When a secondary light source formed on the reticle side focal surface of the fly-eye lens is considered in a case where the light source comprises a laser such as an excimer laser having a spatial coherence, the irradiation light beams corresponding to the lens elements have some considerable amount of coherence from each other. As a result, random interference fringes (speckle interference fringes) are formed on the surface of the reticle or the surface of the wafer which is in conjugate with the surface of the reticle, causing the illuminance uniformity to deteriorate. When its spatial frequency is considered here, a Fourier component corresponding to the minimum interval between the lens elements is present in main. That is, the number of combinations of light beams contributing to the interference is the largest. Therefore, fringes having a relatively low frequency (having a long pitch) in comparison to the limit resolution and formed to correspond to the configuration direction of the lens elements are observed on the surface of the reticle or the surface of the wafer. Although the formed interference fringes have low contrast because the KrF excimer laser has a relatively low spatial coherence, the interference fringe acts as parasite noise for the original pattern. The generation of the interference fringes causes a problem when the illuminance uniformity, which will be further required in the future, is improved. In the case where the annular zone irradiation method is considered, the aforesaid noise concentrically superposes in the vicinity of the limit resolution, and therefore the influence of the noise is relatively critical in comparison to the ordinary irradiation method (see FIG. 14).